In conventional magnetic recording, thermal instabilities of the stored magnetization in the recording media can cause loss of recorded data. To avoid this, media with high magneto-crystalline anisotropy (Ku) are required. However, increasing Ku also increases the coercivity of the media, which can exceed the write field capability of the write head. Since it is known that the coercivity of the magnetic material of the recording layer is temperature dependent, one proposed solution to the thermal stability problem is heat-assisted magnetic recording (HAMR), wherein high-Ku magnetic recording material is heated locally during writing to lower the coercivity enough for writing to occur, but where the coercivity/anisotropy is high enough for thermal stability of the recorded bits at the ambient temperature of the disk drive (i.e., the normal operating or “room” temperature of approximately 15-30° C.). In some proposed HAMR systems, the magnetic recording material is heated to near or above its Curie temperature. The recorded data is then read back at ambient temperature by a conventional magnetoresistive read head. HAMR disk drives have been proposed for both conventional continuous media, wherein the magnetic recording material is a continuous layer on the disk, and for bit-patterned media (BPM), wherein the magnetic recording material is patterned into discrete data islands or “bits”.
In a typical HAMR write head, light from a laser diode is coupled to a waveguide that guides the light to a near-field transducer (NFT) (also known as a plasmonic antenna). A “near-field” transducer refers to “near-field optics”, wherein the passage of light is through an element with subwavelength features and the light is coupled to a second element, such as a substrate like a magnetic recording medium, located a subwavelength distance from the first element. The NFT is typically located at the air-bearing surface (ABS) of the air-bearing slider that also supports the read head and magnetic write pole and rides or “flies” above the disk surface. NFTs are typically formed of a low-loss metal (e.g., Au, Ag, Al, Cu) shaped in such a way to concentrate surface charge motion at a notch or tip located at the slider ABS when light is incident. Oscillating tip charge creates an intense near-field pattern that heats the recording layer on the disk. The magnetic write pole is then used to change the magnetization of the recording layer while it cools. Sometimes, the metal structure of the NFT can create resonant charge motion (surface plasmons) to further increase intensity and disk heating. For example, when polarized light is aligned with an E-antenna type of NFT, an intense near-field pattern is created at the notch or tip of the E-antenna. Resonant charge motion can occur by adjusting the E-antenna dimensions to match a surface plasmon frequency to the incident light frequency. A NFT with a generally triangular output end, sometimes called a “nanobeak” type of NFT, is described in US 2011/0096639 and US 2011/0170381, both assigned to the same assignee as this application. In this type of NFT an evanescent wave generated at a surface of the waveguide couples to surface plasmons excited on the surface of the NFT and a strong optical near-field is generated at the apex of the triangular output end.
In a HAMR disk drive excessive heating of the NFT can cause diffusion of the NFT metal until the NFT tip rounds and recording degrades. One possible cause of failure due to excessive heating may be due to adsorption of carbonaceous material on the slider overcoat near the NFT tip. Hydrocarbon molecules from the disk overcoat and lubricant can become mobile at elevated temperatures and adsorb on the slider ABS. Over time the molecules can form a “smear”, which absorbs power from the NFT and becomes very hot. The hot smear wears out the overcoat, and once the overcoat is gone the heat is transferred from the smear to the NFT, resulting in diffusion of the NFT metal until the NFT tip rounds and recording degrades.
Application Ser. No. 14/255,088 filed Apr. 17, 2014 and assigned to the same assignee as this application, describes an optically-transparent protective film in a window region of the recording-layer facing surface of the slider. The window region surrounds both the NFT output end and the write pole end. In one embodiment the overcoat, which is typically diamond-like carbon (DLC), is located between the NFT output end and the protective film. In other embodiments, which preserve the smooth topography of the ABS, there is no overcoat covering the write pole end so only the protective film in the window region protects the write pole end.
What is needed is a HAMR head that has the NFT output end protected from excessive heating and the write pole end protected by the slider overcoat, wherein the slider overcoat retains a smooth topography at the ABS.